1. Field of the Invention
The invention relates to an apparatus for determining the concentration of a conductive fluid present in a fluid filled borehole, and in particular to devices known as water hold-up meters that are used in oil, gas and water filled bore holes.
2. Related Art
When drilling for and extracting hydrocarbons such as oil and gas, water can also enter the well and flow in the bore hole or drill pipe. It is desirable to understand the relative proportions of water and hydrocarbons in the well, so that the well yield can be understood, and informed decisions taken about well operation and maintenance. If a cross section of the well bore perpendicular to the well axis is considered then the proportion of the cross sectional area occupied by water in relation to the total cross sectional area is known as the water hold-up. Gas hold up or oil hold up can be calculated in a similar manner.
Water and hydrocarbons do not readily form a solution. Instead, the smaller constituent fluid appears as globules within the majority fluid. The globules may be very small, as in an emulsion, or be very large resulting in total separation into layers, or alternating flow known in the art as slug flow. In a pipe line or bore hole that is non-vertical, the lighter fluids will tend to be more concentrated along the upper side of the pipe or hole. Lighter fluids will also tend to flow faster in an upward direction than the heavier ones, even to the extent that particular fluids may move in the opposite direction to the general flow. This is illustrated by way of example in FIGS. 1a and 1b. FIG. 1a is a longitudinal cross-section through a pipe or bore hole showing the lighter hydrocarbons such as oil rising rapidly against the downward flow of water. FIG. 1b is a transverse cross-section, through the pipe shown in FIG. 1a, showing the lateral separation of water and oil, as well as a layer in which globules of water are present in the oil, and globules of the oil are present in the water.
A resistance based water hold up meter works by sensing the apparent resistance of the fluid in the bore hole or drill pipe at an array of points across the area of the bore hole. Any water present will generally contain sufficient salts to make it significantly lower in resistivity than the hydrocarbons. The hydrocarbons on the other hand have a very low conductivity, and will appear mostly insulating. By measuring the resistance at different points across the bore hole, a clearer view of the proportion of water to hydrocarbons can be obtained. Furthermore, the monitored resistances as they vary with position and time, can be interpreted to improve understanding of the composition of the fluid in the bore hole.
U.S. Pat. No. 5,736,637 discloses a known device for evaluating the multiphase flow of fluid down-hole in a borehole.
U.S. Pat. No. 3,792,347 describes the use of largely insulated needle probes to spear oil globules such that the small exposed tip of the electrode loses its electrical path to ground through a predominantly water based fluid while within the globule, leading to a determination of the proportion of oil in the fluid.
U.S. Pat. No. 3,009,095 similarly describes using the resistive property to detect water globules in a predominantly oil based fluid by positioning two electrodes close to each other such that small globules of water between them creates a conductive path.
In order to generate sufficient or reliable data describing the fluid composition in the bore hole, it is desirable to take measurements continuously at a plurality of locations across the bore hole. The rate at which individual measurements of resistance are made by an individual resistance sensor is often several thousand times per second, and a single device often has a plurality of sensors. During the deployment of the device in the bore hole, a large amount of data can therefore be collected, subsequently needing to be processed and stored. Although the data can be stored in the device for later analysis, it is preferable to transmit the data to the surface for immediate analysis so that real time monitoring and decision making can occur. This allows the device to make a second pass through an area of interest immediately, rather than returning the device to the area after completion of an entire run. Transmission of data of this magnitude is effected using a wire-line connection, an electronic connection common in the art, between the surface and the device deployed in the bore hole. The limited capacity of the wire-line to transmit data therefore acts like a bottle neck in the amount of data that can be passed from the device to the surface. Alternatively, the data may be recorded in-situ in the borehole and similarly the capacity of the recording medium may represent a bottle neck between the instrument and the eventual presentation of the data.
In order to maintain sufficient measurement range and resolution it is typical that the measured resistance will be measured across at least 16 binary digits on a logarithmic scale. Given that a resistance value will typically be represented as a 16 bit number (giving approximately 66,000 possible resistance values) and assuming that the sampling occurs at a frequency of say 5 kHz, the wire-line connection from the device to the surface will need to transmit 80 kbs of data per sensor. For a device having 12 sensors, a wire-line connection supporting a bandwidth of nearly 1 Mb/s is then required. However, typical transmission links have a limited bandwidth perhaps in the order of 25 kb/s to 100 kb/s, and often shared with other instruments. We have therefore appreciated that there is a need for an improved device allowing more efficient collection and processing of the water hold-up data to work with commonly available transmission links.